The prostaglandin synthetase inhibitor, indomethacin was administered to hypertensive patients in doses of 150 mg/day during 109 and 9 mEq/day sodium intake. It caused pronounced decreases in plasma immunoreactive prostaglandin E (58-76%), and plasma renin activity (53-89%), and lesser decreases in excretion of aldosterone (25-45%), kallikrein (10-11%), kinins (4-13%) and methoxyhydroxyphenylglycol (6-10%) but had no effect on blood pressure.